1. Field of the Invention
The present invention relates to an exposure control apparatus for a camera having a focus detection device in which an exposure can be precisely determined by detecting backlight.
2. Description of the Related Art
In a conventional camera having a multi-point object-distance measuring device, the focusing operation upon an object to be taken is carried out by employing, for example, the smallest object distance (closest object) selected from among object distances obtained by a plurality of distance measuring areas.
The brightness of the object is calculated based on an algorithm for a predetermined photometering system, such as an averaged overall photometering system, a center-weighted photometering system, or a spot photometering system, using the measurements of the object brightness obtained in a plurality of split photometering areas (the number of which is usually larger than the number of the distance measuring areas) which are partly overlapped on the distance measuring areas. To detect backlight, the difference in the measurement between the central photometering area and the remaining photometering area(s) is calculated, wherein the difference is checked to determine whether the difference is above a predetermined value in order to determine the presence of backlight. Namely, the focusing is carried out based on the measurement of the object distance in one distance measuring area selected from the plurality of distance measuring areas, and the detection of backlight is carried out based on the measurements of the plurality of photometering areas, made independently from the selected distance measuring area.
However, in such a known exposure control apparatus for a camera, a plurality of distance measuring areas are partly overlapped on a plurality of photometering areas. However, the detection of backlight for the object to be taken can be inaccurate since the judgment is carried out based on the plurality of photometering areas, independent of the distance measuring area selected for the focusing operation. Consequently, an accurate exposure value cannot be obtained. For instance, assuming that there are first to sixth photometering areas and first to third distance measuring areas which overlap the fourth, fifth and sixth photometering areas, respectively, the distance data from the first distance measuring area is selected to focus on the object. If photometering data from the sixth photometering area is used as the main measurement of the object brightness, and if the difference in the main measurement between the sixth photometering area and other photometering area (for example, the second photometering area) is above a predetermined value, it is judged that the object includes backlight. In this judgment, the photometering data of the fourth photometering area corresponding to the first distance measuring area whose distance data is utilized, is not used to detect backlight, and hence it is impossible to judge whether the main object within the first distance measuring area (i.e., the object located within the distance measuring area for which the focusing should be carried out) is actually subject to backlight.